The pyrolysis of hydrocarbons is performed in a tubular cracking furnace industrially. As well known, theoretically the chemical reaction of the pyrolysis of hydrocarbons is a strong endothermal reaction, including a primary reaction and a secondary reaction. General speaking, the primary reaction relates to reactions in which big hydrocarbon molecules become smaller molecules, i.e., linear hydrocarbons are dehydrogenated and chain broken, and naphthene and arene are dehydrogenated and ring broken, thus ethylene and propylene and the like are produced in the primary reaction. The secondary reaction relates to reactions in which the products of the primary reaction, namely, olefins and alkynes, are performed to polymerization, dehydrogenating condensation, as well as naphthenes and aromatics are performed to dehydrogenating condensation and dehydrogenating fused cyclization and so on. The secondary reaction would not only greatly decrease the yield of target products, but also produce coke seriously. The coke would deposit on the inner wall of radiant tube. The formation of coke on the inner wall of the radiant tube is greatly disadvantageous for the regular operation of cracking furnace. The coke adhered on the inner wall of the radiant tube would increase heat conducting resistance and stream resistance of reactant fluids in whole reactive system. The increase of both heat conducting resistance and stream resistance will be against primary reaction.
Industrially, cracking furnace decoking has to be performed periodically due to the coking on cracking furnace. The interval between decoking is called “run length”. Usually, at the end of the every “run length”, due to the coke layer, tube metal temperature (TMT for short) would tend to exceed the maximum (generally 1125° C.) of tube material requirement.
Therefore, it will help to lengthen the “run length” and increase the cracking furnace's processing load, if the coking in the cracking furnace is suppressed. To suppress coking, it is necessary to decrease the secondary reaction as much as possible while maintaining the primary cracking reaction in radiant tube. Therefore, it should be avoided to unnecessarily heat the product of the primary reaction above the highest temperature of cracking temperature range and to retain excessive reaction time in the radiant tube. In addition, a contrary restrict factor is that lower pressure is helpful for the primary reaction, since pyrolysis is a reaction of volume increasing.
Chinese patent CN1133862C discloses a twisted-tape tube (please see attached FIGS. 4 and 5), wherein said twisted-tape tube is arranged in the radiant tube at regular intervals. The operating principle of “twisted-tape tube” could be described briefly as follows: As is well known, heat transfer process of radiant section in ethylene cracking furnace may include following steps. At first, the gas inside hearth transfers heat into the outer wall of radiant tube through radiation and convection, and then the outer wall transfers heat to inner wall and the likely existent coke layer by wall heat conduction, finally heat is transferred to internal fluid from inner wall by convection. According to the boundary layer theory of Prandtl, when the fluids flow along a solid wall surface, a thin fluid layer near the wall surface will be adhered on the tube wall surface without slipping, thus a flowing boundary layer is formed. Because the boundary layer transfers heat by conduction, its heat resistance is very high although the boundary layer is very thin. Then heat is transferred to the center of turbulent flow through the boundary layer by convection. According to above analysis, the most resistance of tube heat transfer is on the boundary layer and the coke layer adhered on tube inner wall surface. If the resistance by the boundary layer could have been reduced, heat transfer efficiency will be greatly intensified. The twisted-tape tube in CN1133862C is developed base on such principal. The twisted-tape tube arranged in the radiant tube will force to change fluids flow from plug flow to turbulent flow. Thereby the fluids will have a strong traversing flush effect on the tube wall, thus the boundary layer will be destroyed and got thinner. As a result heat transfer resistance nearby flowing boundary layer is decreased, and heat transfer efficiency is intensified.
In this invention the “twisted-tape tube” and related members are all called with general name of “heat transfer intensifying member”, this term refer to all members arranged in the radiant tube that be able to force to change fluids from plug flow to turbulence flow and thus to destroy and thin the boundary layer. It is not only restricted to “twisted-tape tube”.
Although heat transfer between radiant tube and inner fluids could be intensified by arranging twisted-tape tube and alike member, it does not necessarily mean the more the better. The reason is that, when the members are arranged in the radiant tube, the pressure drop would be increased accordingly in tube. Also as mentioned above, the pressure drop increase is adverse to perform the cracking reaction.
Therefore considering tube pressure drop, the twisted-tape tube could not be arranged as more as possible. This invention is to address this confliction, i.e. to arrange certain number of twisted-tape tubes to maximize heat transfer and restrain coking at the farthest, thus to greatly enhance processing load and extend run length before decoking.